One conventional technique for processing of cracked naphthas involves performing a selective hydrodesulfurization of the cracked naphtha. A selective hydrodesulfurization refers to a process where sulfur is removed from the naphtha while minimizing the amount of olefin saturation that occurs in the reaction. Avoiding olefin saturation is valuable, as it leads to a higher octane naphtha product. Retaining a higher octane value allows a selectively hydrodesulfurized feed to be used as a naphtha fuel stock without having to use a reforming step.
The catalysts used for a selective hydrodesulfurization process typically include a combination of a Group VI metal and a Group VIII metal on a suitable support, such as a catalyst including cobalt and molybdenum on an alumina support. A number of compounds have previously been identified as reaction inhibitors for selective hydrodesulfurization catalysts. These reaction inhibitors reduce the activity of the catalyst for performing hydrodesulfurization.
U.S. Pat. No. 2,913,405 describes a process for desulfurizing a cracked feed to sulfur levels below 0.03 wt % sulfur. The process is described as providing better olefin retention for feeds that include a sufficiently large amount of nitrogen. Several examples are provided of adding a constant amount of nitrogen during a hydrodesulfurization process that is performed at a constant temperature.
U.S. Patent Application Publication No. 2003/0220186 describes a process for treating a catalyst to improve the selectivity of the catalyst for hydrodesulfurization relative to hydrogenation. The catalyst is first exposed to a protective agent, such as CO or ethanolamine. The exposure to the protective agent is maintained while the catalyst is also exposed to a concentration of olefinic species substantially greater than the amount of olefins present in any typical feed. After both the olefinic species and the protective agent are removed from the feed, the hydrodesulfurization activity of the catalyst will be mostly restored, while the hydrogenation activity will remain at a substantially lower level.